1. Field of the Invention
The present invention relates to a free space optics communication apparatus which is provided at each of two spaced points in opposition to each other to perform communication by transmitting an optical signal through a light beam propagating in free space, the apparatus having a function of correcting the optical axis of a light beam to deal with an angle change of the apparatus.
2. Description of Related Art
A free space optics communication apparatus which generally propagates a light beam in free space to perform communication needs to transmit a narrow light beam with a minimized divergent angle in order to efficiently transmit the power of light. A narrower light beam, however, is susceptible to swinging due to wind pressure or vibrations in a building or a support member, distortion because of temperature changes, angle variations due to changes over time and the like, so that the light beam is likely to miss a target apparatus and stable communication is difficult to achieve. To address this, as shown in FIG. 13, a proposed apparatus has a function of correcting optical axis displacement by correcting an angle change of the apparatus, if any, to direct the light beam to a target apparatus at all times.
FIG. 13 shows a schematic diagram of a free space optics communication apparatus which performs optical communication with a target apparatus, not shown. In FIG. 13, reference numeral 100 shows an optical system for transmitting and receiving a light beam. Light for transmission to the target apparatus is emitted from a light emitting element 210 such as a semiconductor laser. The light for transmission emitted from the light emitting element 210 is polarized and the polarization direction is set to be parallel with the paper. The light polarized in this direction is reflected by a polarization beam splitter 220 toward a light transmission/reception lens 230 which changes the light into a generally collimated light beam 240 with slight divergence before the beam 240 is transmitted toward the target apparatus.
On the other hand, received light transmitted from the target apparatus reversely follows the path on the same optical axis as that of the transmitted light from the free space optics communication apparatus and is incident on the polarization beam splitter 220 through the light transmission/reception lens 230. Since the polarization direction is set to be orthogonal to that of the transmitted light (the polarization direction is perpendicular to the paper), the light passes through the polarization beam splitter 220 and enters a beam splitter 250.
Most of the received light is reflected by the beam splitter 250 and is incident on a light receiving element 260 for light signal detection to detect a signal for communication. Some of the received light, however, passes through the beam splitter 250 and is incident on a light position detecting element 270.
The light position detecting element 270 is realized, for example, by a photodiode divided into four as shown in FIG. 14A. FIG. 14A shows a light spot 42 applied to the photodiode divided into four from 27a to 27d. 
The light position detecting element 270 outputs a signal in accordance with the distribution of the light intensity in the spot formed on a light receiving surface, and the outputs from the four photodiode portions 27a to 27d are compared. The position of the light spot 42 can thus be found.
Another type of the light position detecting element is a special photodiode for detecting the position of a light spot, generally called a PSD as shown in FIG. 14B, for example. For the PSD, the vertical position of the light spot 42 can be found by comparing a voltage across terminal Y1 with a voltage across terminal Y2, while the horizontal position of the light spot 42 can be found by comparing a voltage across terminal X1 with a voltage across terminal X2.
The signal output from the light position detecting element 270 is arithmetically processed as angle correction information by a control circuit 280 and a drive signal is output to a drive circuit 290 for the optical system 100. The drive circuit 290 drives a drive mechanism 300 which drives the optical system 100 in a vertical direction and a drive mechanism 310 which drives the optical system 100 in a horizontal direction to correct the optical axis such that the position of the light spot 42 is shifted to the center of the light position detecting element 270. In the example shown in FIG. 14A, the optical system 100 is driven in a direction in which all the outputs from the four photodiode portions 27a to 27d are equal.
All of the light position detecting element 270, the light emitting element 210, and the light receiving element 260 for light signal detection have been subjected to position adjustments to align their optical axes. When the light spot 42 is applied to the center of the light position detecting element 270, the received light is also incident on the center of the light receiving surface of the light receiving element 260 for light signal detection, and the center of the light from the light emitting element 210 is directed toward the target apparatus. In this manner, the correction of optical axis displacement is performed such that the transmitted light is sent in the direction of the received light, that is, toward the target apparatus, at all times.
When the light position detecting element 270 is used for position detection, the output sensitivity to a position change depends on the area of the spot 42 formed on the light receiving surface in the example of FIG. 14A. It is desirable for the light receiving spot 42 to have an appropriate area in order to prevent a sudden change in the output when any border between the four photodiode portions 27a to 27d is crossed and to provide proper position sensitivity.
In the example shown in FIG. 14B, since the size of the spot for which operation is guaranteed is defined by limitations in the specifications of the PSD, the area of the spot on the light receiving surface cannot be reduced significantly. Thus, the light receiving surface of the light position detecting element 270 is generally set at a position shifted from a light convergent point.
The free space optics communication apparatus which transmits/receives light through the air in the aforementioned conventional example, however, is affected by the phenomenon of the light beam waving due to fluctuations of the air. The air fluctuations are classified broadly into two, that is, macro fluctuations which cause the waving of the whole light beam, and micro fluctuations which cause non-uniform distribution of intensity within a transmission beam. Of these air fluctuations, the macro fluctuations of the air can have less influence by increasing the diameter of a transmission beam to some extent at a transmission point, providing an automatic tracking mechanism, and the like.
FIG. 15 shows modeled micro fluctuations of the air. In a transmission path between the free space optics communication apparatus and the target apparatus, the refractive index is changed over time and has non-uniform distribution because of mixing of the air with different pressures and temperatures and the like.
For this reason, a portion W1 with high intensity and a portion W2 with low intensity are produced in a transmission beam area W. In addition, the intensity of the light beam at a point in the space is changed over time. Thus, the portion W2 with low intensity is observed as if it waved randomly in the transmission beam area W and the waving is called the micro fluctuations of the air.
The conventional free space optics communication apparatus having the light axis displacement correcting function is formed such that the light receiving surface of the light position detecting element 270 is disposed at a position shifted from a light convergent point. When the micro fluctuations are present in the air, the spot 42 formed on the light receiving surface has non-uniform distribution of light intensity.
As shown in FIG. 16, the light with non-uniform intensity distribution at a beam port (M in FIG. 15) of the apparatus corresponding to an entrance pupil is projected as it is, so that a portion with low intensity and a portion with high intensity are produced, and the center of the light intensity, different from the center of the luminous flux, is determined as the optical axis.
As a result, as shown in FIG. 16, even when the central point of the spot 42 is located at the center of the light receiving surface and no displacement of the optical axis occurs actually, the optical axis is corrected such that the spot 42 is shifted to the lower left due to the non-uniform intensity distribution. Since the optical axis direction is displaced by the angle corresponding to that position shift, the error in correcting the optical axis displacement is increased, and at worst, transmitted light does not reach the target apparatus to disable communication.